linux-kernel - Re: [PATCH] KVM: SVM: drop MSR_IA32_PERF

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Date:   Tue, 16 Jun 2020 09:52:54 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Like Xu <like.xu@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: SVM: drop MSR_IA32_PERF_CAPABILITIES from emulated MSRs

On Tue, Jun 16, 2020 at 9:45 AM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Jim Mattson <jmattson@...gle.com> writes:
>
> > On Tue, Jun 16, 2020 at 9:14 AM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> >>
> >> state_test/smm_test selftests are failing on AMD with:
> >> "Unexpected result from KVM_GET_MSRS, r: 51 (failed MSR was 0x345)"
> >>
> >> MSR_IA32_PERF_CAPABILITIES is an emulated MSR indeed but only on Intel,
> >> make svm_has_emulated_msr() skip it so it is not returned by
> >> KVM_GET_MSR_INDEX_LIST.
> >
> > Do we need to support this MSR under SVM for cross-vendor migration?
> > Or, have we given up on that?
>
> To be honest I'm not sure about the status of cross-vendor migration in
> general and PMU implications in particular, hope Paolo/Sean can shed
> some light. In this particular case my shallow understanding is that
> MSR_IA32_PERF_CAPABILITIES has only one known feature bit which unlocks
> an MSR range with additional counters. If the feature bit is not set
> this, I guess, can easily be migrated (basically, let's allow writing
> '0' there on AMD and return '0' on read). But what if the feature was
> enabled? We'll have to support the new MSR range and do something with
> it after migration (run intel_pmu in fully emulated mode?).
>
> Anyway, the immediate issue I'm trying to fix here is: whatever is
> returned by KVM_GET_MSR_INDEX_LIST can be successfully queried with
> KVM_GET_MSRS as some userspaces count on that.

That's a nice property. Is it documented somewhere?

Reviewed-by: Jim Mattson <jmattson@...gle.com>